Gene localization is done by in situ hybridization of radioactive RNA to human chromosomes. Our previous work has located about half of the 5S rRNA genes on the long arm of chromosome one. Current research is concentrating on mapping the other 5S rRNA genes on the remainder of the genome. New and improved technical advances will permit the detection of gene sequences at lower levels of DNA repetition than were previously possible. There is an interesting correlation between the human DNA satellites, II and III with the chromosomes which have the ribosmal precursor genes. Interphase nuclei with genetic variant "C bands" are being examined to understand the role of these DNA satellites in positioning the ribosomal genes during interphase when the nucleolus is making ribosomes. Before attempting to map specific tRNA species on human chromosomes we will isolate and purify tRNA in a large scale, selecting a few major species for high purification and RNA:DNA hybridization on nitrocellulose filters. These and other parameters are required before we can proceed in precise gene localization.